TimeTrack от carcinizer

TimeTrack allows you to track the amount of time you spend on the internet. It displays a circle in a toolbar that indicates the remaining time. You can create multiple groups to monitor different activities such as checking news, social media, work or playing music in the background. You can also (optionally) set the addon to block sites after the time is up.

Permissions:

Access browser tabs
Required to retrieve information about currently active tabs.

Download files and read and modify the browser’s download history (optional)
Required when exporting addon's data.

Access your data for all websites (optional)
Required to display timeout popup when a Block on timeout option is enabled in any group.